It is per se known to determine an estimated position for a vehicle, to calculate the limit error of the estimated position occurring due to error in the estimated position and error in the road map, to extract the links that are within the range of the limit error of the estimated position, to implement map matching to these links, and thus to calculate the position of the vehicle (refer to Patent Literature #1).
With this prior art technique of Patent Literature #1, by extracting the links while considering, not only error in the estimated position of the vehicle, but also error in the link data, it is possible to implement map matching to the links and also to determine whether the vehicle has deviated from a road, according to error in the estimated position of the vehicle and also error of the links.